Catalytic dehydrogenation can be used to convert paraffins to the corresponding olefin, e.g., propane to propene, or butane to butene. U.S. Pat. No. 5,481,060 discloses an exemplary dehydrogenation process. In a typical arrangement for a catalytic dehydrogenation, the process includes a reactor section, a catalyst regeneration section, and a product recovery section. The product recovery system includes various zones to remove one or more contaminants from an effluent from the reaction section.
For example, the effluent from the reactor section typically passes through a chloride removal section. After chloride removal, the treated effluent is passed to a reactor effluent dryer system (RED) for drying and further purification, including removal of water and hydrogen sulfide (H2S). An exemplary reactor effluent dryer (RED) system includes two or more adsorbent beds arranged in a typical thermal swing adsorption (TSA) system.
As is known, in a TSA, while one or more adsorbent beds is operated in adsorption mode to purify and dehydrate the process stream, the other bed(s) are operated in regeneration mode. When the adsorbent bed(s) in the adsorption step starts to breakthrough the contaminants, the bed(s) on adsorbent mode is switched to regeneration mode and the freshly regenerated bed(s) are placed in adsorption mode. The beds are switched between adsorption and regeneration modes to provide for continuous purification of the process stream. Regeneration of the adsorbents is accomplished by purging the beds with a regenerant stream such as an inert gas, net gas, or vaporized hydrocarbon stream, at elevated temperature to desorb the impurities and water to rejuvenate or regenerate the adsorbent and prepare it for a fresh adsorption step. The TSA process is well known to those skilled in the art.
After desorbing and/or regenerating the regenerable adsorbed in the RED, the spent regenerant gas is typically cooled down and passed to a collection drum to remove heavier hydrocarbons (formed in the reactor through side reactions), such as polynuclear aromatics. The cooled gas is then passed into a regenerant gas scrubber, and, after being cleaned, depending on the composition of the regenerant gas, the regenerant gas may be used as fuel gas.
The regenerant gas scrubber typically contains circulating caustic solution (sodium hydroxide (NaOH)) in which the hydrogen sulfide (H2S) is converted into sodium sulfide (Na2S) and sodium bisulfide (NaHS). Both of these sulfide compounds are toxic and presents environmental problems. In addition, the caustic solution is considered spent at approximately 70% utilization. The disposal of the spent caustic solution is costly and creates handling problems. Furthermore, since the caustic solution has to be continuously replaced, the operating costs associated with constantly supplying caustic and disposing of same can be very large. The use of solid potassium hydroxide (KOH) pellets placed in a vessel may not address these problem because of operational difficulties associated with the solid particles, and the continue problems associated with spent material disposal.
Alternatively, it is known to treat regenerant gas in a sulfur recovery unit (SRU) which utilizes the Claus catalytic process which converts hydrogen sulfide to elemental sulfur. This treatment of the regenerant gas without utilizing caustic is possible for large facilities processing hydrogen sulfide containing waste streams from different units. However, the dehydrogenation units are typically part of a petrochemical complexes which rarely have an SRU. Thus, the petrochemical complexes typically resort to utilizing caustic.
Therefore, there remains a need for an effective and efficient process for treating a spent regenerant gas that does not utilize a caustic solution or solid hydroxide salt pellets and that does not require an SRU. It would also be desirable to have such a process that allows for the regenerant gas to be recycled instead of being used as fuel gas.